The present invention relates to a method for manufacturing an even laminated product by extruding a molten thermoplastic coating-material track onto a track of basic material. The invention also relates to an apparatus for applying the method.
When molten polymer is extruded through a narrow-slitted die onto a track of material which serves as the bearing surface a shrinkage or puckering occurs during cooling due to the internal forces of the polymer, and therefore the final polymer layer on the track is considerably narrower than the die slit. Another consequence of the puckering phenomenon is that the edges of the molten polymer layer are many times thicker than the rest of the polymer layer. The puckering and edge-thickening phenomena cause torsion, thickening of the reel edges, and breakage of either the polymer track or the other tracks in the final product reel. These detrimental phenomena are eliminated in known coating and laminating technology by widening the die through which the melt is extruded to such an extent that the thick polymer edges extend over the bearing track, or by cutting off the track edge strips consisting of coating and basic materials immediately after the coating process and before the final reeling. Also the edges of the reels with the coating extending beyond the edges must be cut in a further treatment, in which case a problem is caused by the coating material sticking to the press roll.
In spite of great efforts to eliminate the puckering and thickening phenomena in the treatment of molten polymer (e.g., shaping of the die slit, air jets, various mechanical knives, coextrusion dies, hot wires, etc.), the cutting of the edge strip cannot be eliminated by existing methods. This cutting for its part, usually results in the strips being wasted. Such waste makes the extrusion coating applications considerably less economical since the strips consist of several layers which cannot be separated and then reused.
If the basic track is of a fibrous material, such as paper or cardboard, and if the tracks could be separated, even if with difficulty, fibers are always left in the polymer strip and prevent its reuse for extrusion. On the other hand, fibrous strips cannot be broken up in a pulper, owing to the polymers present in them, although attempts have been made to develop the method. If, again, the basic track has been some plastic or other film, the layers have usually adhered to each other so that their separation is not profitable. Therefore the strips are usually taken, after cutting and chopping, to a baling station and then to a dump or an incinerator.